The Bragg peak therapy of cancer patients with heavy charged particles requires precise localization of the Bragg peak on a tumor volume. A slight error can cause severe underdosing of the tumor region and overdosing of the nearby critical organs. For precise localization of the Bragg peak, one requires and experimentally measured value of the water equivalent thickness between the point of entry and the target volume. Between these two end points there can be unknown amounts of bone, tissue, sinus, air etc. Presently used CT techniques may not be as reliable as desired, especially when there is a large amount of thick bone or air path. We have a technique which uses high energy radioactive beam and a sensitive positron camera, which, we expect, will achieve the desired accuracy. In this technique, radioactive particles such as 19Ne or 11C are used. Both of these particles decay by emitting positrons, which then annihilate with the medium electrons to produce two gamma rays, separated by 180 degrees. These gamma rays can then be detected in coincidence mode by a positron camera to locate their origin in space. Through adjustment of the energy of the radioactive particles (mostly 19Ne will be used), they will be stopped on the location of the tumor, verifiable by the positron camera. This method provides a direct measure of the required water equivalent thickness. This technique will be applied in the treatment position (on-line) and just before the actual therapy. Unlike X-rays (used in CT), radioactive particles have penetration properties that are very similar to those of the heavy charged particles used in Bragg peak therapy. the value of the water equivalent thickness measured by a radioactive beam is then directly applicable for treatment planning with heavy charged particles. Precise delivery of Bragg peak dose on a tumor volume, while sparing the normal cells and hearby critical organs, constitutes and extremely important component of the evaluation of heavy particle treatment modality. This is a long term goal of the laboratory.